feat: add organization scoping to chats (#23827)

Fixes https://github.com/coder/internal/issues/1436

* Adds organization_id to chats with backfill (workspace org → user org membership → default org)
* No support yet for ACLs (follow-up issue)
- Cross-org workspace binding rejected (both in `CreateChatRequest` and in `create_workspace` tool
- Adds `OrganizationAutocomplete` to `AgentCreateForm`
- Docs updated with `organization_id` in chats-api.md

> 🤖 Written by a Coder Agent. Reviewed by many humans and many agents.

---------

Co-authored-by: Mathias Fredriksson <mafredri@gmail.com>

coderd/database/queries/chats.sql

@@ -392,6 +392,7 @@ LIMIT
 
 -- name: InsertChat :one
 INSERT INTO chats (
+    organization_id,
     owner_id,
     workspace_id,
     build_id,
@@ -406,6 +407,7 @@ INSERT INTO chats (
     labels,
     dynamic_tools
 ) VALUES (
+    @organization_id::uuid,
     @owner_id::uuid,
     sqlc.narg('workspace_id')::uuid,
     sqlc.narg('build_id')::uuid,